Totally understand your feeling! My last 'yan yan' was exactly like yours, couldn't understand both Cantonese and English, everyday I was like a mad woman! As a result, I fired her after 4 1/2 months. Now my new helper has been with me two months, she speak good English and Cantonese, thank God I only need to say once whatever I want her to do!!
I think your option is to find a replacement as soon as you can, don't even think she can be improved!
Good luck!!